400 mgs a week is about the standard dosage for a 200 lb guy. It is recommended for people who are not advanced to use appx 2 mg per lb of bw a week, ergo 400 mgs a week for a 200 lb dude. I've seen people put on 20-30 pounds on a sensible 12 week cycle, with deca obviously there is less water retention so it would be a little harder to add that much weight unless he had some test or something in the mix, but it's not unheard of. If I were to do a cycle like that, I would probably be eating 20 - 22 times bodyweight in lbs worth of calories each day which is enough to gain appx 20-25 lbs over the course of 12 weeks. Obviously some of that would be fat.
His strength increases do seem kind of extreme though and I would question whether those are legit or not. Seems like you would injure yourself adding that much to your bench or squat in such a short period. I realize that deca keeps the joints feeling good and maybe that would help a bit but deca does not make your joints invincible.